1. Field of the Invention
The present invention relates to a minute magnetic flux measuring method and apparatus for embodying the same, which are adaptable for, for example, a leakage flux flaw detection.
2. Description of the Related Art
A method for detecting a flaw of a steel pipe, a steel plate, etc., may come in three varieties, an ultrasonic flaw detecting method, an eddy current flaw detecting method, and a leakage flux flaw detecting method. Of those methods, the leakage flux flaw detecting method is relatively widely used, because it can detect flaws in both sides of a thick steel plate from a single side, and it has a high sensitivity in detecting flaws in the innards of a steel pipe.
A typical known technique of the leakage flux flaw detecting method will be described. As shown in FIG. 1, a DC power source 3 supplies a DC power to a coil 2 wound around a magnetizing yoke 1. A test piece 4 to be detected is put on the magnetizing yoke 1 and is magnetized there. If the detected test piece 4 contains a flaw 5, a magnetic flux partially leaks outside of the test piece 4 from the flaw 5, as indicated by dotted lines. A magnetic flux sensor 6 detects the leakage flux and converts it into an electrical signal. In this way, the flaw 5 is detected in an indirect manner. The leakage flux emanating from the flaw 5 is very weak as shown in FIG. 2, which graphically illustrate relationships between the leakage flux and a distance between the surface of a steel material and Hall device as the magnetic flux sensor. FIG. 3 shows a steel material to be used for a measurement in FIG. 2. In the figure, W indicates the width of the flaw and "d", the depth of the flaw.
From this fact, it is seen that the requirements for the magnetic flux sensor 6 for detecting the leakage flux are: 1) The sensitivity of the magnetic flux sensor for a weak magnetic field is high. 2) The initial bias voltage of the sensor little varies. 3) The temperature characteristic of the sensor is good.
A sensitivity of the magnetic flux sensor now marketed, however, is very small as shown in FIG. 4. In the figure, a line "a" indicates a sensitivity of a magneto-diode as a magnetic flux sensor; a line "b", that of a magneto-resistive sensor; a line "c", that of a Hall device.
A variation of the initial bias voltages of twelve magneto-resistive flux sensors is as shown in FIG. 5. Even if a nondefective portion (free from any defect) of the test piece 4 is magnetized, a leakage flux is detected from the nondefective portion. This leakage flux is called as initial bias voltages. As see from the graph, the initial bias voltages of the sensors are greatly different from one another. Therefore, before use, the initial bias voltage must be adjusted for every sensor; otherwise when the detected signal is amplified, the amplifier using the sensor of the low initial bias voltage is saturated, and consequently the flaw detector is inoperative for flaw detection.
A temperature characteristic of a magneto-diode 7 has been obtained, as platted in FIG. 7, when the output voltage of the magneto-diode 7 is measured by using a circuit in which a magneto-diode 7 is connected through a resistor 8 to a DC power source 9, as shown in FIG. 6. As seen, a rate of change of an output voltage of the sensor against temperature is great.